There is a proposal (see for example Japanese Patent Application Laid-Open (JP-A) No. 2004-219967) for a broad band terahertz wave generator that by adopting a cascade configuration, employing a common reflection mirror for a first oscillator and a second oscillator that oscillate with different wavelengths, as an optical parametric oscillator for 2-wavelength oscillation, implements stable 2-wavelength oscillation in which even when one wavelength is changed, there is no change imparted to the oscillation state of the other wavelength. A terahertz wave measurement apparatus is also proposed equipped with such a terahertz wave generator and a room temperature operating detector.
There is also a proposal (see for example JP-A No. 2009-276389) for a spectrometer in which: signal light and pump light from a pump light source are multiplexed by a nonpolarization beam splitter to generate multiplexed light, this is amplified by a fiber amplifier, and then the multiplexed light is divided in two to multiplexed light A and B by a polarization beam splitter, and the multiplexed light A and B are mixed by a polarization beam splitter so as to overlap in a GaP crystal at the phase matching angle which they form. Terahertz waves are thereby radiated in two directions, with a terahertz wave from the difference frequency mixing of the pump light in the multiplexed light B with the signal light in the multiplexed light A and a terahertz wave from the difference frequency mixing of the pump light in the multiplexed light A with the signal light in the multiplexed light B. One of the terahertz waves that are radiated in the two directions can thereby serve as reference light.
There is also a proposal for a terahertz wave generator capable of generating terahertz waves with different spectral characteristics using a single optical crystal (see, for example, JP-A No. 2012-177896). In the terahertz wave generator of JP-A No. 2012-177896, an optical crystal is configured by sticking together a DAST crystal and a DASC crystal such that their a axes are aligned. A terahertz wave caused by the DAST crystal is generated when excitation light is made incident to the DASC crystal side, and a terahertz wave caused by the DASC crystal is generated when excitation light is made incident to the DAST crystal side. Terahertz waves with different spectral characteristics can thereby be generated by a single optical crystal by switching the incident direction of excitation light incident to the optical crystal.